Conventional alkaline electrochemical cells are formed of a cylindrical housing having an open end. The housing is initially formed with an enlarged open end. After the cell contents are supplied, the cell is closed by crimping the housing edge over an edge of the end cap assembly and radially compressing the housing around the assembly to provide a tight seal. The end cap assembly comprises an exposed terminal end cap plate and typically a plastic insulating member which forms a plug at the open end of the housing and insulates the terminal end cap plate from the cell housing. A problem associated with design of alkaline cells is the tendency of the cell to produce gases as it continues to discharge beyond a certain point, normally around the point of complete exhaustion of the cell's useful capacity. Alkaline cells are conventionally provided with a rupturable member such as a diaphragm or membrane within the end cap assembly, for example, as described in U.S. Pat. No. 3,617,386. Such diaphragms or membranes are designed to rupture when gas pressure within the cell exceeds a predetermined level. The end cap assembly may be provided with vent holes for the gas to escape when the diaphragm or membrane is ruptured. The end cap assembly disclosed in U.S. Pat. No. 3,617,386 takes up considerable space, which reduces the amount of available space within the cell for active material.
In order to provide a tight seal alkaline cells have end cap assemblies which include a metal support disk inserted in a cavity within the plastic insulating member. The metal support disk is a separate member apart from the terminal end cap plate. The metal support disk may have a convoluted surface as shown in U.S. Pat. Nos. 5,532,081 or 5,080,985 which assures that end cap assembly can withstand high radial compressive forces during crimping of the cell's housing around the end cap assembly. Such support disk allows high radial forces to be applied during crimping. This results in a tight mechanical seal around the end cap assembly at all times. Such designs, however, can occupy additional space within the cell and can markedly increase the complexity of fabrication, particularly if applied to very small cells.
U.S. Pat. No. 4,670,362 discloses a plastic insulating disk which is snap fitted into the open end of a cylindrical casing for an alkaline cell. The disclosed insulating disk does not comprise nylon. The insulating disk disclosed in this reference is not contemplated for use in very small size cells, e.g., AAAA alkaline cells. The disclosed snap fitted insulating disk requires additional force for insertion into the open end of a cylindrical casing. Any additional force required for insertion of the insulating disk becomes a disadvantage in sealing very small diameter cells, e.g., AAAA size alkaline cells, because of the difficulty in handling such smaller cells.
A rupturable vent membrane may be integrally formed as part of the plastic insulating member included within the end cap assembly. Such vent membrane may typically be of circular shape as shown in U.S. Pat. No. 4,537,841. This reference discloses a discontinuous skirt emanating from the base of the insulating member and requires a separate metal support disk held in place within a cavity in the insulating member. As shown in this reference the rupturable membrane may be integrally formed as a thin portion of the plastic insulating member. The rupturable membrane may also take the form of a grooved or circumferential configuration as disclosed in U.S. Pat. No. 5,080,985. Methods of crimping the housing around the insulating member is disclosed in U.S. Pat. No. 5,150,602.
A problem associated with the design of end cap assemblies for AAAA alkaline cells is that any plastic insulating disk employed to seal the cell is of necessity very small diameter corresponding to the small cell diameter. Because such disks are very small, they may be more prone to cracking than plastic insulating disks for larger size cells, particularly when radial compressive forces are applied.